bug 1682: r6312 was producing a MySQL error (depending on the MySQL server
version) because a count() implies a group by.
This code change was checked against MySQL 5.0.75, MySQL 5.0.51 (where the
error occured) and SQLite 3.6.22.
git-svn-id: http://piwigo.org/svn/trunk@6322 68402e56-0260-453c-a942-63ccdbb3a9ee